Planar diaphragm electromagnetic loudspeakers are generally popular because of their good sound reproduction characteristics. Such loudspeakers typically include a generally flat diaphragm composed of a membrane having a pattern of one or more conductors thereon which form the "voice coil" or signal current carrying conductors. The membrane is positioned so that the conductors are located close to and in opposed relation to magnets, or a magnetic sheet, so that the conductors are attracted and repelled by the magnets as current signals pass through the conductors, thereby causing the membrane to oscillate and produce sound.
A typical planar diaphragm includes a thin flat membrane of MYLAR with a pattern of thin foil-like conductors on the membrane. Aluminum is a popular conductor material because of its light weight. This provides a rugged construction capable of withstanding high power input and transients without damage, since the resistance of the conductors is quite low and the area of heat dissipation of the conductors is quite large.
Planar diaphragm loudspeakers have good sound reproduction characteristics, particularly in the low frequency range. However, in order to produce low frequency sound of reasonable amplitude, it is necessary for the diaphragm to move relatively large amounts of air, which in the past has required high power input and a large diaphragm and thus, a correspondingly large speaker, because of the relatively short excursion or front to back movement of the diaphragm.
The short excursion resulted from the need to maintain the diaphragm at least slightly tensioned in its mid-plane position so it could not ripple or wave like a flag, and thus cause objectionable sound distortion. To avoid such ripple movement distortion, the diaphragms in the past have been tensioned, and then rigidly attached to a surrounding frame. Thus, the extent of excursion or movement at the center of the diaphragm was quite limited, because of the required low elasticity of the diaphragm material. A diaphragm with a large area was therefore needed, and high power input was required for high amplitude low frequency response, because of this short permissible excursion of the diaphragm.
Other problems encountered in the past with planar diaphragms related to the difficulty of obtaining full range sound reproduction, without the use of multiple separate diaphragms for low, mid-range, and high frequencies. Such separate diaphragms increase the cost of the loudspeaker construction, and usually require in addition, cross-over networks, and in many instances, matching impedences for one or more of the diaphragms. Such separate diaphragms have been used in the past because a relatively thick membrane diaphragm with relatively heavy conductors which are required to accurately reproduce low frequencies, has only fair mid-range response, and very poor high frequency response. Similarly, a thin membrane with light conductors to obtain good high frequency response cannot move sufficient air to reproduce the low frequencies, and a diaphragm with good mid-range response is usually deficient in both the bass and treble range.
Another problem with dipole loudspeakers, such as planar diaphragm speakers, is the need to space these speakers from a room wall behind the speaker to avoid reflections which distort the sound emanating from the front of the speaker.
A further problem has been distortion of different frequencies of sound produced by planar diaphragm loudspeakers.